Increase the max upload filesize in apache/php

Edit your php.ini.  where is my php.ini?

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; Maximum allowed size for uploaded files.
upload_max_filesize = 2M

Change the 2M to whatever new value you want, then restart apache.

Note, this setting may also be related:

; Maximum size of POST data that PHP will accept.
post_max_size = 8M

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Note, some RoundCube versions have limits set in the file

/var/www/html/roundcube/.htaccess

so be sure to increase them there as well, if they're set.